1. Field of the Invention
The present invention relates to a connecting construction of a flat panel display device such as a liquid crystal display device and a drive circuit substrate thereof and, particularly, to a flat panel display device having a construction, in which a display panel provided with a plurality of TCP's (Tape Carrier Packages) is easily detachably mounted on a casing of the flat panel display device, and a manufacturing method thereof.
2. Description of the Prior Art
As a flat panel display device, a liquid crystal display device, a plasma display device and an electroluminescence device, etc., are well known and, particularly, the liquid crystal display device has been used widely as a display of a mobile information terminal, a notebook type personal computer, and a home use word processor, for which the portability is required.
As a typical example of the flat panel display device, the liquid crystal display device will be described. With the recent tendency of reduction of size and weight of an electronic device on which a liquid crystal display device is mounted, a further reduction of size of the liquid crystal display device is required. Particularly, reduction of an area of the liquid crystal display device except an image display region thereof, that is, a frame portion of the liquid crystal display device, and reduction of thickness of the liquid crystal display device itself are important problems to be solved.
In order to drive the liquid crystal display device, semiconductor driver elements each in the form of such as LSI, etc., called “driver LSI's” are required. In order to achieve the reduction of size of liquid crystal display device, a COG (Chip On Glass) method for directly forming the driver LSI on a display panel substrate has been developed. However, if the driver LSI directly formed on the display panel substrate is defective, the whole panel substrate becomes defective. In order to solve such problem, it has been usual to arrange a driver LSI on each TCP (Tape Carrier Package) by using TAB (Tape Automated Bonding) so that the driver LSI's are connected to a display panel substrate through the TCP's.
The TCP manufactured by using the TAB includes a plurality of lead electrodes, which are connected to protruded electrodes, called “bumps”, of the LSI called “bare chip”. The lead electrodes on the side of the display panel or the circuit substrate are called as “outer leads” and those on the side of the LSI are called as “inner leads”. The lead electrodes comprise a number of stripe-shaped electrodes formed by patterning a copper foil formed on an insulating base film of such as polyimide, etc. In order to highly reliably connect the LSI by contacting the bumps of the LSI to the inner leads under pressure, the base film is made thick enough to provide a sufficient mechanical strength thereof during the pressure-contact step.
Therefore, when the TCP having one end connected to the display panel is bent to fit it to a suitable holding member provided on a chassis of the display device, the bending operation is facilitated by forming a slit or slits in only a portion or portions of the TCP at which the latter is bent to form a rounded portion thereof. Japanese Patent No. 2616564 and Japanese Patent Application Laid-open No. H9-265104 disclose examples of a liquid crystal display device employing TCP's each having such slit or slits and a flexible portion.
Particularly, in the liquid crystal display device disclosed in Japanese Patent Application Laid-open No. H9-265104, a liquid crystal panel is held by a lock member such as protrusions provided on a chassis and circuit substrates each connected to a plurality of TCP's are held by hook portions provided on a side surface of the casing. With such construction, an assembling and disassembling of the liquid crystal display device are facilitated.
When this construction in which the circuit substrates and the display panel are merely held in positions without using adhesive, there is a merit that, when a liquid crystal panel becomes failure, an exchange of the liquid crystal panel with a new panel is facilitated.
However, in order to hold the circuit substrate by the hook portions, it is necessary to pull up the circuit substrate and then insert them into insides of the hook portions. In a case where circuit substrates are fixed to side surfaces of a chassis by adhesive, it is enough to merely bend TCP's at slit portions thereof so that a length of each TCP may be minimum. In the hooking type construction, which requires the pull-up step of the TCP's and the insertion step thereof into the hook portions, the length of each TCP is increased by an amount necessary for pulling the TCP up. Further, a peeling stress is generated in the connecting portion between the display panel and the TCP when the latter is pulled up. In addition, since the increased length portions of the TCP's are risen up from a plain of the connecting portion after the circuit substrate is hooked in the hook portions, it is necessary to provide a space for receiving the risen portion of the TCP.
Since the liquid crystal display device is usually used as a display of a portable information terminal, a notebook type personal computer or a home-use word processor, etc., the liquid crystal display device has to be not only highly compact but also durable against vibration and thermal shock. On the other hand, since, when the size of liquid crystal display panel is increased, the liquid crystal display panel may be deformed due to thermal expansion of the panel itself, the adhesive is usually not used to fix the liquid crystal panel to the chassis. Instead thereof, the liquid crystal panel is supported by only the hooking members as mentioned previously. The size of the liquid crystal panel and the positioning of the hooking members mounted on the chassis are determined by taking the fact that there are unavoidable machining errors (about several tenth millimeters each) of the liquid crystal panel and the chassis into consideration. That is, in order to improve the product yield of the flat panel display device, it is usual to design the size of the liquid crystal panel and the positioning of the hooking members mounted on the chassis such that the length of the hooking members mounted on the chassis is slightly larger than an ideal size of the liquid crystal panel in order to allow the liquid crystal panel to be reliably fitted in the chassis. Therefore, in a finished liquid crystal display device, there may be a case where the liquid crystal panel slightly moves with respect to the casing.
With such movement of the liquid crystal panel with respect to the casing, a stress is exerted on the TCP's connected to the liquid crystal panel, so that there is a possibility that a connection in the pressure-contacted portion between the circuit substrate and the TCP's and a connection in the pressure-contacted portion between the liquid crystal panel and the TCP's are broken. It is difficult to absorb such stress by only the slit provided in only the rounded portion of each TCP.
In the technique disclosed in Japanese Patent Application Laid-open No. H9-265104, a rounded portion of a TCP is received between a frame and a liquid crystal panel by pulling a flexible portion of the TCP up to a vicinity of an upper surface of the liquid crystal panel and then bending it down such that the rounded portion of the TCP has a large radius of curvature even after a circuit substrates are held by hook portions of a chassis.
Therefore, in view of prevention of peeling-off due to bending stress of the TCP caused by reduction of the size of liquid crystal display device, the provision of the rounded portion having large radius of curvature is meaningful. However, in view of further reduction of thickness and size of the flat panel display device, the structure has to be improved further. Further, in the construction, in which the TCP has such large rounded portion, the total length of the TCP is large. However, the length of the TCP should be as short as possible in view of high frequency characteristics of the display device.
For a reference purpose only, the above mentioned breaking phenomenon of the connection will be described with reference to FIGS. 1A to 1C, which show the steps of mounting a liquid crystal panel and a circuit substrate on a casing. A liquid crystal panel 1 is held on a main surface of a casing 4, as shown in FIG. 1A. Then, each TCP 2 is bent at a slit 7, which is provided in a region thereof to be rounded, along a ridge portion of the casing as shown in FIG. 1B and a circuit substrate is held in a hook portion 6 provided on the side surface of the casing 4 as shown in FIG. 1C.
As shown in FIG. 1B, in holding the circuit substrate 3 in the hook portion 6, it is necessary to pull up the rounded portion of the TCP's 2. When the TCP's 2 is pulled up in such manner, loads acting in opposite directions to peel off the TCP's 2 from the liquid crystal panel 1 are exerted on the junctions between the TCP's 2 and the liquid crystal panel 1, with which lead lines in the junctions may be broken.
Furthermore, in pulling the TCP's 2 up, loads are also exerted on connecting portions between the TCP's 2 and the circuit substrate 3, with which the connection between the TCP's 2 and the circuit substrate 3 may be broken.
The above-mentioned problems may also occur in other flat panel type display devices.